1. Field of the Invention
The present invention relates to a photomask used in photolithography to transfer an image to a photoresist layer on a substrate such as a semiconductor wafer. In particular, the present invention relates to a phase shift mask and to a method of repairing a phase shift mask.
2. Description of the Related Art
A photomask is an optical element through which a photoresist film, formed on a semiconductor substrate, is exposed to light of a given wavelength (referred to hereinafter as “exposure light”). A typical photomask, known as a binary mask, has a light-shielding pattern formed on a transparent quartz substrate. The light-shielding pattern is made of chromium so as to block light. Thus, the exposure light that is not incident on the light-shielding pattern is transmitted through the photomask to expose respective areas of the photoresist film and thereby transfer an image corresponding to the light-shielding pattern onto the film. The photoresist film is subsequently subjected to a developing process which removes either the exposed or non-exposed portion of the film and thereby forms a photoresist pattern.
Nowadays, the manufacturing of highly integrated semiconductor devices requires the forming of very fine patterns on the substrates. However, the fineness of the pattern that can be attained using a binary mask is limited due to certain optical effects associated with the mask. Accordingly, a phase shift mask capable of forming a finer photoresist pattern than a binary mask is currently in wide use in the semiconductor manufacturing industry.
A phase shift mask has a phase shift pattern constituted by a material, e.g., molybdenum suicide, that transmits a portion of the exposure light, as opposed to material such as chromium that blocks all of the incident exposure light. Such a phase shift mask is disclosed in detail in U.S. Pat. No. 5,286,581.
As is well known by those skilled in the art, forming a fine photoresist pattern using a phase shift mask requires a light source that emits exposure light of a relatively short wavelength. Currently, a KrF laser that emits ultraviolet light having a wavelength of 248 nm or an ArF laser that emits light having a wavelength of 193 nm has been used as the light source. Furthermore, the energy of light is inversely proportional to its wavelength. Accordingly, when the photomask is irradiated by light of a short wavelength, the high energy level of the light causes impurities in the photomask to cohere. The impurities include sulfur (S), carbon (C), and ammonia (NH3), those created as by-products of the process of manufacturing the photomask, and nitrogen (N2) and water vapor (H2O) which are introduced from the ambient of the room in which the photolithography process is being carried out.
The cohered impurities block a portion of the light during the exposure process. Consequently, the photoresist film may be patterned undesirably at areas corresponding to the locations of the cohered impurities. Thus, the photomask should be cleaned to remove any lumps of cohered impurities. The cleaning process is usually performed by the manufacturer of the photomask not by the end user, i.e., the semiconductor chip manufacturer.
However, a phase shift pattern of, for example, molybdenum silicide, is etched by an etchant used in the cleaning process, as shown in FIG. 1. More specifically, with reference to FIG. 1, a typical phase shift mask includes a photomask substrate 10 that is divided into a main region b and a peripheral region a, and a phase shift pattern 11b exposing portions of the photomask substrate 10 within the main region b. Most of the peripheral region a is covered by a phase shift film 11a and a light-shielding film 12. The phase shift pattern 11b within the main region b is produced by etching the phase shift film 11a. At this time, the thickness of the phase shift pattern 11b is substantially identical to the thickness to of the phase shift film 11a. 
However, as mentioned above, the phase shift pattern 11b is etched during the cleaning process. As a result, the thickness of the phase shift pattern is reduced to t during the cleaning process (t<to). Furthermore, the thickness t of the phase shift pattern 11b is a technical factor that determines the light transmittance T of the phase shift pattern. The light transmittance T of the phase shift pattern is, in turn, a characteristic of the mask that affects the process conditions and process margin of the wafer exposure process. Still further, the cleaning process may be repeated throughout the course of manufacture of the phase shift mask. As a result, the thickness of the phase shift pattern 11b is further reduced.
FIG. 2 is a graph of the results of an experiment in which a phase shift mask was cleaned several times. Originally, the phase shift pattern had a light transmittance of about 8%. Then, the phase shift mask was cleaned several times. Accordingly, the light transmittance of the phase shift pattern 11b increased linearly with the number of cleanings, as shown in the graph. The light transmittance of the photomask exceeded the limit transmittance Tc of 9% after the phase shift mask was cleaned 8 times. The limit transmittance Tc is a threshold value at which the photomask is no longer useful for a particular wafer exposure process.